The treatment of surfaces of metallic articles includes such processes as carburizing, hardening, nitriding, bluing, blackening, controlled oxidizing and/or controlled reducing.
In particular, carburizing may be defined as the introduction or application of additional carbon to the surface of a ferrous metal article to increase the carbon content of the surface, and to some limited depth, beneath the surface (the depth of substantive penetration of the carbon hereinafter called “case”) of the article. When the article is subsequently subjected to an additional heat treatment, the surface portion carburizes resulting in a substantially harder surface than the underlying virgin or “green” metal. This is known in the art as “hardening.”
There are a number of processes that have been used to carburize ferrous articles. Perhaps the earliest application is “box carburizing” where open charcoal pits were used. Bone meal was packed around the articles to provide a protective atmosphere when heated and to be the source of carbon. That process has evolved into “pack carburizing” where articles to be carburized are packed into a box with a carburizing compound, such as metal carbonates burned to a hardwood charcoal by the use of oil, tar and the like, packed thereabout. Carbon is formed on the surface of the steel by the decomposition of carbon monoxide (from the carburizing compound) into carbon and carbon dioxide. The carbon dioxide that is formed reacts immediately with the uncondensed carbon in the carburizing compound to produce fresh carbon monoxide. This process is repeated as long as there is enough carbon present to react with the excess of carbon dioxide and until the surface of the ferrous article is saturated. This “class” of carburizing requires a solid carburizing compound “packed” about the article.
Another process which is used is liquid carburizing in which the steel or iron is placed in a molten salt bath that contains chemicals such as barium cyanide and the like required to produce a chafe comparable with one resulting from pack carburizing. The piece is placed in the bath for a predetermined length of time at elevated temperature such that the carbon diffuses into the surface of the metal. This “class” of carburizing is distinguished from the prior art to which this invention relates by its requirement for a liquid or salt bath into which the article is submerged.
Another process is “gas” carburizing in which a gas containing carbon is used as a gaseous material to provide gas phase carbon atoms to iron to produce the face centered iron with carbon in the matrix as well as iron carbide (Fe3C) precipitate. Gas carburizing can be further divided into atmosphere gas carburizing and vacuum carburizing with vacuum ion carburizing as a separate species of vacuum carburizing.
Atmosphere gas carburizing is a well-developed technology that has proven acceptable for most case hardening carburizing applications. In atmosphere gas carburizing, a lower hydrocarbon typically natural gas (methane), propane or butane, is metered into an endothermic gas furnace atmosphere maintained at positive pressure (i.e., at “atmospheric” pressure) in an industrial furnace. By controlling the dew point of the gas composition (endothermic gas and carburizing gas), most typically the CO/CO2 gas ratio (water gas shift reaction), the gas carbon potential is controlled. Typically, the gas carbon potential is below the saturation of carbon in the iron solution and when sufficient carbon in the iron matrix and iron carbide (Fe3C) precipitates are formed throughout the surface, the gas carbon potential of the furnace atmosphere gas is changed to lower value (“equilibrium carburizing”) to allow the carbon to diffuse into the case. The diffusion can be controlled vis-à-vis gas composition and temperature. For example, it is quite easy with atmosphere gas carburizing to actually decarb (remove carbon from) the surface during diffusion to allow a harder article composition between article surface and “green” core (portion of virgin metal beneath surface not affected by carburizing) because the case depth is increasing during diffusion.
Further, in atmosphere gas carburizing the carbon potential does not have to be set at saturation limits of the steel. Specifically, the carbon potential can be set at lesser values to avoid a natural phenomenon occurring at saturation referred to herein as “carbide network.” That is, at saturation, the surface of the article comprises iron carbides closely packed as adjacent molecules of face centered carbon steel that can be viewed as linked together in a “carbide network.” When carbon diffusion occurs it is potentially possible that groups or clusters of the packed iron carbide molecules are not homogeneous throughout the case. Conventional metallurgical thinking in the trade is that over time and at high stress, the carbide network can function as a stress riser. Some metallurgists, however, do not share this opinion.
With atmosphere gas carburizing, the carbide network can be minimized by controlling the carbon potential to minimize the formation of the network in the first place. That is, if carburizing does not occur at saturation, the network is not likely to be formed. In the past, atmosphere gas carburizing produced metal oxides on the article surface because of the presence of oxygen in the atmosphere. For this reason, atmosphere gas carburizing is fundamentally different from vacuum gas carburizing which does not have oxygen.
In the past, several atmospheric furnaces used butane and air, in which it is believed that the CO2 raises CO levels and also leaks out the furnace to prevent sooting. It is also believed that water additions may have the same effect and boost hydrogen without boosting CO. When unsaturated aliphatic hydrocarbons break down during carburizing, they often produce a byproduct known as soot which includes solid carbon particles. The soot collects in the furnace during the carburizing process and must be removed. This requires extra maintenance and expense to keep the operation clean and reduces productivity. In the past, the higher order hydrocarbons especially had a tendency to deposit soot.
Carburizing typically occurs immediately upon introduction of the carburizing material into the furnace chamber. However, for an atmospheric carburizing furnace to work, it must first be “seasoned.” Seasoning is the process of putting carbon into the furnace brick and alloy. “Green” furnace brick and alloy will provide a non-equilibrium carburizing environment for the work pieces. Until the furnace is seasoned, the work pieces will be low in carbon content and case hardness. The controllability of the process is therefore a function of the sensitivity of atmosphere carbon sensors and the ability of the gas flow metering valves to meter the gas. In order to control carbon potential, one must measure CO2 level, dew point level, or oxygen content. Measurements of the gas composition in the furnace chamber are usually taken. One gas is measured and the enriching gas flow is controlled to produce a below saturated iron carbide surface that was subsequently diffused into the case. Where an additional nitrogen bearing gas was used (ammonia for carbonitriding), the additional gas was set to a fixed quantity.
For mixtures of the higher order unsaturated or saturated aliphatic hydrocarbons (which unsaturated hydrocarbons are highly reactive, such as, for example LPG) the process that is used to pressurize and deliver the gas to the furnace can affect the composition of the gas metered into the furnace. Depending on the purity of the feedstock and the gas delivery system, variations in the hydrocarbon makeup can occur. While there may be some cracking of the hydrocarbons in the delivery system that will not materially alter the carburizing process (since the hydrocarbon must be reacted anyway to produce the carbon by keeping the reaction going forward), in practice, variations do exist in the gas composition delivered to the furnace, causing imprecise control and variation in the repeatability of the process.
Atmosphere gas carburizing technology often uses a “Class 302” atmosphere, which is typically prepared by mixing a readily available hydrocarbon such as methane (natural gas) or propane with a greatly reduced amount of air than would be used for normal combustion. By definition, “Class 301” and “Class 302” (“lean” and “rich”, respectively) endothermic atmospheres are formed by partial reaction of a mixture of fuel gas and air in an externally heated catalyst filled chamber. A “Class 301” atmosphere is generally defined as a “lean” endothermic and has a typical final product of: 45% N2, 19.6% CO, 0.4% CO2, 34.6% H2, and 0.3% methane. A “Class 302” atmosphere is generally defined as a “rich” atmosphere and has a typical final product of: 39.8% N2, 20.7% CO, low trace amounts of CO2, 38.7% H2, and 0.8% CH4. For Class 302 atmospheres, the current technology endothermic gas generators produce a mixture typically defined as 40% H2, 40% N2, 19.6% CO, 0.3% CO2 and 0.1% methane, based on natural gas. These chemistries change slightly depending on natural gas content, relative humidity of the incoming air and overall conditions of the catalyst and the generator itself, as well as natural gas make-up.
A protective atmosphere can be used in a Class 302 endothermic gas atmosphere for hardening processes; typically, in the 1500 to 1650° F. range; for carburizing, typically in the 1500 to 1850° F. range; or for carbonitriding typically in the 1500 to 1640° F. range.
In a conventional Class 301 or 302 endothermic generators, a mixture pump draws fuel gas to air mixture that is maintained by way of a gas regulator and air/gas mixer. The pump forces the mixture to enter the bottom (typically) of the reaction tube. The reaction tube, heated externally by flame or electric heating element, maintains a reaction tube (retort) temperature typically of 1900° F. to 2000° F. The inside of the reaction tube is filled with a nickel based catalyst where the air/gas mixture converts to the 40% N2, 40% H2, 20% CO mixture. Upon completion of the reaction, the product gas is quickly cooled to freeze the reaction. Cooling of the gas occurs by either a water cooler or air cooled heat exchanger. The first 20% of the retort tube is filled with an Allundum sphere which provides heating of the air/gas mixture, but does not promote chemical reactions. At the conclusion of the heating of the gas, the nickel catalyst is reached and a two-stage reaction occurs. The first stage combusts the air in the mixture, generating N2+H2O+CO2+heat+excess feed. The second stage, in combination with heat supplied to the retort tube from external means, drives the CO2 level down and creates CO. Likewise, excess fuel causes the H2O to drive down to H2.
At such time, the mixture reaches the end of the reaction retort, the gas is chilled and frozen to the constituents mentioned earlier. Based on variations that occur in feed gas, variations in final CO level can occur causing variation in product gas carbon potential content. The present invention provides a control system to regulate these variations.
It is to be noted that changes alone, even though regulated, can cause changes in the final metallurgical outcome. Thus, elimination of the fluctuations would improve metallurgical results in demanding applications.
It is also to be noted that, in certain articles of the world, natural gas or propane may not be available. In these areas, high purity liquid hydrocarbons may be available. In such areas of the world and even in the United States, the feed gas supplies have high sulfur content. Sulfur levels, over 10 ppm, can damage the nickel catalyst in as little as a few hours. Having a high purity liquid hydrocarbon can eliminate the sulfur problem.
Typically, natural gas will yield a hydrogen level twice that of carbon monoxide. In gas carburizing, the CO content is the main vehicle for carrying carbon to the work piece. The byproducts of carburizing are oxygen and hydrogen. Having an atmosphere that is higher in CO and lower in hydrogen will be of benefit to the speed of the carburizing reaction. It is typical for carburizing to raise the carbon potential to 0.80% or higher. To do so, an enrichment gas, typically propane or methane is added to the furnace. During this time, the demand for carbon by a green article will consume the enrichment gas. As carbon consumption occurs, CO2 and water vapor levels rise and conversely carbon potential falls.
To lower the CO2 and water vapor levels, an enriching gas may be added. There are three primary reactions: CH4+CO2 becomes 2 CO+2H2, and likewise, H2O+CH4 becomes CO and 3H2. When equilibrium is achieved at a given carbon potential set point, likewise CO2 and H2O levels will also be stabilized in equilibrium. The third reaction is temperature dependent and CH4 goes to carbon into the work surface and 2H2 is generated. Note that there are other reactions that occur with oxygen, and that to drive the reaction forward, hydrogen is generated from all three reactions as a resultant. The control of enriching gas is typically done by a motorized valve operator or a time proportioned on/off solenoid valve.
One of the problems associated with methane specifically is that the excess amounts of hydrogen generated dilute the otherwise desired CO. Since methane has two H2's per molecule of carbon, an abundance of hydrogen is generated and a likewise fall in CO occurs, more so with this hydrocarbon than the other, higher order, hydrocarbons. In certain cases, CO has been known to fall to as low as 15%. This is known as “CO depletion.” The net result is that the carbon potential is difficult to achieve and methane levels rise due to the fact that insufficient CO is available to react with the methane and raise the carbon potential. The above effect is most prevalent in high surface area loads. In certain instances, the described “CO depletion” effect is less severe with propane gas due to the reduced hydrogen to carbon ratio.
In an atmosphere carburizing or hardening furnace, a Class 302 atmosphere is piped to the furnace or an atmosphere is created by flowing nitrogen, typically, or air in conjunction with a liquid-dripped into the furnace, such as methanol or acetone. Since a Class 302 atmosphere is 40% hydrogen, 40% nitrogen, and 20% CO, the Class 302 atmosphere has a carbon potential typically between 0.20% carbon and 0.45% carbon. Depending on the type of hydrocarbon used, the percentages of these three main components changes somewhat. In the past, there have been applications where a liquid is used for “enrichment” of the process. The amount of enrichment liquid used was very low and often, control of such small amounts of liquid was difficult to meter. In the past, variable speed pumps or very small motorized needle valves were used.
The vast majority of furnace Class 301 and Class 302 atmospheres are created using endothermic gas processes, as discussed above. However, there is a competing process that was very popular in the late 1970's and early 1980's when natural gas curtailment took place. During this era the alternate atmosphere using gaseous nitrogen and liquid methanol was fed directly into the furnace. A typical gas usage was 160 cfh nitrogen and one gallon per hour methanol. The combined nitrogen and methanol yielded an equivalent atmosphere of 400 cfh of 40% N2, 40% H2 and 20% CO. The nitrogen methanol atmosphere does not require a generator and also does not rely on natural gas. For areas of the world where natural gas is not available, nitrogen methanol atmospheres are typically used.
The nitrogen is stored nearby outside as liquid and evaporated from liquid into a gas before entering the furnace. The methanol is also stored outdoors in liquid form and pumped into the furnace. The liquid methanol is dripped into the furnace, traditionally from overhead at a rate to yield the 40% H2 and 20% CO levels. Upon dripping into the furnace, the methanol flows through a “sparger” pipe. Inside the furnace, the sparger pipe has a solid end and has a series of small holes drilled around its circumference. The liquid methanol cannot exit the end of the sparger since it is blocked, but most exits the small holes as a vapor.
However, there are several problems with this technology. First, the sparger is intended to vaporize the methanol. Liquid methanol dripping directly onto a workload will stain the load and yield non-uniformity in the carburized case. The end of the sparger is closed, forcing the vaporized methanol to leave through the small holes. As the sparger ages, the end of the sparger often falls off, causing the above problems. Second, due to the high carbon content of the methanol, the sparger holes become plugged with carbon causing the ratio of methanol to nitrogen to become “lean.” This results in an extremely poor carburizing case or complete loss of the carburized case. Third, the one gallon per hour flow rate is very low. If the rate varies slightly lower, carbon potential can easily fall 10 to 20 points and an oxygen probe or CO2 carbon control system will go into error. The workload will be undercased, low in surface carbon, and low in hardness, which are considered very undesirable. Should the flow rate be too high, the reverse will happen and there is potential for carbide networks in the workload and the furnace can easily become sooted.
All of these situations are considered very undesirable. It is noted that one gallon per hour is approximately equal to 2.1 liquid ounces per minute or 1 ml/sec. The control valves used to meter this liquid rate are very difficult to control with the desired accuracy needed to prevent the problems discussed above.
Further, when the sparger becomes plugged, there is a loss of the carrier gas for the furnace, causing the furnace pressure to drop to unsafe levels. Since the spargers are prone to plugging, this condition makes the furnace potentially unsafe.